Re: [PATCH 2/4] ARM: at91: debug_ll: make UART base address configurable

From: Ahmad Fatoum <a.fatoum@pengutronix.de>
To: Sam Ravnborg <sam@ravnborg.org>
Cc: barebox@lists.infradead.org
Subject: Re: [PATCH 2/4] ARM: at91: debug_ll: make UART base address configurable
Date: Tue, 19 Feb 2019 22:10:02 +0100	[thread overview]
Message-ID: <7820ced1-0a40-9bad-85ac-f3711161b96a@pengutronix.de> (raw)
In-Reply-To: <20190219204233.GA14204@ravnborg.org>

Hello Sam,

On 19/2/19 21:42, Sam Ravnborg wrote:
> Hi Ahmad.
> 
> On Tue, Feb 19, 2019 at 01:06:10PM +0100, Ahmad Fatoum wrote:
>> This is in line with other platforms such as i.MX, which allow
>> specifying a debug port. As we can't use port indices because
>> the UARTs aren't mapped consecutively, allow specifying a hex
>> base at configuration time.
>>
>> A side effect of this patch is that sama5d4's HAVE_AT91_DBGU2
>> is now honored as well. Previously anything besides DBGU0
>> defaulted to DBGU1.
> 
> This approch hardcoded the DBGU address based on the configuration.
> Can you somehow come up with something that is run-time configured?

I think it should remain build-time configurable, to account for cases
such as when the designated debug port isn't usable for whatever reason
(e.g. its exposed via a USB-to-UART bridge and you want to use a proper
UART, because you hate that the disconnection on power cycle..).

The hard coded values are just some often applicable defaults.

> So we can continue towards the goal to have multi-image support
> for at91.
> The current approch will limit us to a single type of SOC -
> and we may try to combine more than one SOC in the same multi-iamge
> sometime in the future.
> 
> Maybe each board have to call an debug_ll_init(address) function or similar.
> If not called we can use the default which is the common address
> for most at91 SOCs.

It's a low level debug option, my understanding is that you only enable it
when one specific entry point fails to load on your board and you need some early
output to start debugging.
It shouldn't be kept on by default. Indeed only 6 defconfigs out of 126 ARM defconfigs
turn it on. (One of them my fresh microchip_ksz9477_evb_bootstrap_mmc_defconfig, oops..)

For the general task of having a console in PBL, there is pbl_set_putc,
which boards code could configure.

> Or maybe something smarter.
> 
> 	Sam
> 

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